1. Field of the Invention
The present invention relates to an optical recording medium capable of signal recording and regeneration with an optical beam.
2. Description of the Prior Art
As an optical recording medium or a recording layer thereof for use in an optical disk, there are already known a rare earth-transition metal alloy film, a reducible oxide film utilizing phase transition from amorphous state to crystalline state of a charcogenide compound, a heat-mode recording medium, a thermoplastic recording medium etc. For example, a magnetooptical recording medium of a rare earth-transition metal alloy film can be composed of a polycrystalline film such as MnBi or MnCuBi, an amorphous film such as GdCo, GdFe, TbFe, DyFe, GdTbFe or TbDyFe, a single crystal film such as GdIG.
Among these recording medii, the aforementioned amorphous films are recently considered suitable as a photothermal recording medium, in consideration of the film forming property at the preparation of a film of a large area at an almost ambient temperature, the signal recording efficiency for recording a signal with a small photothermal energy, and the signal readout efficiency for reading the recorded signal with a high S/N ratio. Among said materials GdTbFe is suitable for use as a magnetooptical recording medium because of a large Kerr angle of rotation and a Curie point in the range of 150.degree. C. Also the present inventors have found, as a result of investigation for increasing the Kerr angle of rotation, that GdTbFeCo provides a magnetooptical recording medium with a sufficiently large Kerr angle of rotation and enabling signal readout with a high S/N ratio.
However the amorphous magnetic materials employed in the magnetic recording medii, including the magnetooptical recording medii, such as the aforementioned GdTbFe, are generally associated with a drawback of poor anticorrosive property. In contact with air or water vapor, such materials show a deterioration in the magnetic properties therefore and eventually become transparent by complete oxidation. This drawback is commonly found not only in the magnetooptical recording medii but also in the aforementioned optical recording medii. Particularly, in a construction wherein a reflecting layer or an interference layer and a reflecting layer are provided on the back of a magnetic recording layer with a view to improve the S/N ratio, the thickness of the magnetic recording layer is limited to 500 .ANG. or less due to the necessity of effectively utilizing the Faraday effect and therefore, the corrosion resistance is more aggravated. Also, such deterioration of the recording characteristic by oxidation has been a disadvantage common to opto-magnetic recording mediums and optical recording mediums.
In order to avoid such drawback there has been proposed to add an element such as Si, Cr, Ti, Ni, Co to the magnetic recording layer, to cover the recording layer with a transparent protective layer composed for example SiO.sub.2 or SiO, an air sandwich structure in which the recording layer is sealed with an inert gas, or a laminated structure in which another substrate is adhered onto the protective layer by means of an adhesive material, but such structures have been unable to provide a sufficient anticorrosive property.